Trunk piston and method of making same



Feb. 5, 1957 w. M. VENNER ET AL 2,780,505

TRUNK FIsToN AND METHOD oF MAKING SAME Filed Oct. 18, 1952 FIGZ.

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United States Patent O TRUNK PISTON AND METHOD F MAKING SAME William M. Venner, Clayton, Percy L. Bowser, Jr., Ladue, and William F. Peterson, Creve Coeur, Mo., assignors to Sterling Aluminum Products, Inc., St. Louis, M0., a corporation of Missouri Application October 18, 1952, Serial No. 315,519

6 Claims. (Cl. 309-13) This invention relates yto trunk pistons, particularly of a type designed for use in internal combustion engines for automobiles. Such pistons are usually constructed of a metal of relatively high thermal expansion, such as aluminum and magnesium alloys containing various alloying elements such as copper, silicon, nickel, manganese, etc. Where silicon is employed on the order of 12%, the thermal expansion is materially reduced. Copper, nickel and manganese impart hardness and strength to the alloy. The temperatures encountered in internal combustion engines are, however, high, so that there is liability of seizure when hot and of looseness when cold.

A trunk piston of the type described comprises'a head of relatively thick section, a skirt of relatively thin section and wrist pin bosses on and inside of the skirt. The head is grooved to receive the packing rings and is of smaller diameter than the skirt, which latter provides the guiding crosshead of the piston. The skirt is separated from the head in one or both regions of the thrust faces by one or two circumferential slots, and one or two vertical slots may also be provided. The skirt may be of circular contour, or oval with the major diameter in the regions of the thrust faces. The skirt is usually given a standard taper with a smaller diameter at the top.

In a piston of the character described it has been proposed to place a control element in the form of a ferrous ring, viz. of a metal of relatively 'low thermal expansion, Within and extending completely around the top of the skirt. In that proposal the alloy was cast around the ring so -as to surround the same completely around skirt, except to a slight extent where a vertical `slot was formed during casting or Where narrow lugs on the core support the ring during cooling.` In such a construction the top of the skirt will, therefore, have an annulus of the alloy inside as well as outside of the ring, which requires thickening of the skirt even though the ring `be close to Ithe outside of the skirt. This introduces a stiffness into the skirt, even in the regions of the thrust faces and, moreover, the expansion of the skirt is not properly controlled. Furthermore, the special narrow lugs must be provided on the core to position the ring in Ithe mold during the casting operation. These lugs are usually near the boss regions and the recesses are of slight extent. In such a construction, where there is circumferential separation of the skirt from the head in the region of the lowest packing groove, an extra cutting operation is necessary. Where the core has a part extending outwardly to the bottom of the to-beformed packing ring groove, such part cannot be withdrawn from the inside annulus adjacent the ring in a thrust face region.

One of the objects of this invention, therefore, is to provide a trunk piston employing a ferrous control element or ring whereby the expansion of the skirt may be closely controlled.

Another object is to provide a method of casting a trunk piston of the character described whereby the extra operation of circumferential separation of the `skirt from the yhead is eliminated.

ICC

tion, in which will be set out an illustrative embodiment of this invention; it is to be understood however, that this invention is susceptible of various embodiments within the scope of the appended claims.

Referring to the accompanying drawing:

Figure 1 is a vertical section on the wrist pin axis of a piston embodying this invention;

Figure 2 is a section on the lines 2 2, Figure l;

Figure 3 is a section on `the lines 3 3, Figure 2;

Figure 4 is a view similar to Figure 2, but showing the mold and core in and over which the piston is cast;

Figure 5 is a View similar to Figure 3, but showing lthe mold and core and with the core expanded;

Figure 6 is a view similar t-o Figure 5, but showing the core collapsed; and V v Figure 7 is a detail showing another embodiment of this invention.

Referring to the accompanying drawing, and more particularly to Figures 1, 2 and 3; 1 designates the head and 2 the skirt of a piston embodying this invention. The head, which is of relatively heavy section is provided with packing ring grooves 3, while the skirt is provided with inwardly extending wrist pin bosses 4, which are connected to the head `by ribs 5, land with balancing lugs 6. The head is separated from the skirt by one or two circumferential slots 7, inthe regions of the thrust faces and,

in this embodiment, those slots are in the region of the lowest packing ring groove 3. One or more vertical slots 8 may be provided in one or both thrust faces.

A continuous ferrous ring 10, viz., of a metal of rela# tively low thermal expansion, which in the embodiment shown is in `the form of a wire lies within and extends completely around the top of the skirt below the bottom packing groove, close to the outside of the skirt. ln accordance with this embodiment, the ring is enveloped by the alloy in the regions of the bosses and this envelopment extends partially into the thrust face regions. The ring has its inner part exposed circumferentially and radially and medially, viz., to about one-half of its crosssection, along the middle parts of the thrust face regions; however, such exposure may be 4only along one thrust face region. The annulus 11 is recessed at 12 to expose about one-half of the cross-section of the ring along each thrust face. This recess, however, extends upwardly, radially and axially to the bot-tom of the last packing ring groove to separate the skirt from the head in that region when that groove is cut. The `core is so formed with reference to the recess that the core when collapsed will clear the casting and the ring therein.

Referring now to Figures 4-6, which illustrate the method of making a piston which may generally be that as shown in Patent Nos. 2,129,351, 2,676,371 and 2,676,372, 20 designates `the outside mold, which is usually in sections sliding on a base and which is provided with cores 21 to form the holes for the wrist pin bosses. The core is a five-part core, comprising a -three-part mid dle core consisting of parts 22, 23 and 24 and side cores 25, which latter form the wrist pin bosses. Core parts 25 are provided with annular recesses 26 in the regions of Ithe thrust faces and extending into the boss regions to form the annuli 11, and these recesses 26 are interrupted to provide core parts 4i) on the cores 25, 23 and 24, which are recessed at 3@ and form the recesses 12 and expose the inside parts of the ring l0 in the thrust face regions, the ring being supported by the recesses 30 in `the core parts. The core part 23 is also provided with a rib 27 to form the vertical slot 8, and where such vertical slot is in both thrust faces the core part 23 is also provided with such a rib. vThis rib is recessed to free the ring 10 whenV the core part 24 is withdrawn. The core parts 23, 24 and 25 are thus recessed `at 30 to about one-half of the cross- Patented Feb.,5, 1957` section of the ring to the extent of the recesses 12 in order to support the ring on the core. The gate for the mold is shown at 29.

In the casting of this piston, seeiFigs. 4, 5 and 6, the ferrous ring isplaced over the core and within the recesses 30 therein while the core is collapsed, Fig. 6, so that when the. core is expanded, Fig. 5, the ring will lie within the recesses. The mold sections are now closed and the alloy iscast. After the casting has solidified, core part 22 is retracted so as to permit the core parts 23 and 24 to move into the space vacated by the core part 22 and so as to clear the ring 10 and also the vertical slot or slots 8. The core parts 23 and 24 are now also retracted so as to permit the side cores 2S to move into the space vacated 'by the core parts 22, 23 and 24, Fig. 6, in order to clear the wrist pin bosses. This movement of the side cores 25 will also free Vthem from the ring 1) because the circumferential extensions of the recesses 26 and 30 in the side core parts are sufficient to permit such freeing, because the chords of the sidecores are smaller than the respective chordal dimensions of the casting. The mold is nowopened to release the cores 21 and the casting can be picked olf.

In the complete casting the ferrous ring will lie within and kextend completely around the top of the skirt and will b'e surrounded by the alloy inthe regions of the bosses; however, the inner part of the ring is exposed extensively circumferentially and radially along the thrust face region or regions. The continuation of the lenvelopment of the ring 10 into the thrust face regions will serve to somewhat stitfen the ring so that it can be made of small cross sections, while still attaining the mode of operation` as hereinafter described. Furthermore, the parts 40, Fig. 4, on the core parts 23, 24 and 25 are continued circumferentially into the core parts 25 as shown dotted at 41, Fig. 4, above the to-be-formed annuli 11 and circumferentially. beyond the to-be-formed recesses 12, in order to form with the parts 40 the circumferential slots 7 to the bottom of the to-be-formed lower ring groove 3. Accordingly, when this ring groove is cut, the circumferential slots 7 will be formed separating the skirt from the head Without any extra slotting being required. The construction and arrangement of the core parts 22, 23, 24 and 25 is, however, such that they not only form the casting but that they can be readily withdrawn from the castlng.

`When, the alloy is cast on the piston and has solidified, upon cooling from molten to cold condition, the alloy will shrink much faster than will the steel ring so that the alloy will contract and finally arrive at a condition of tension. In so doing theferrous ring is placed under compression radially as well as circumferentially in the condition of the parts shown in full lines, Figure 3. When now the circumferential slots 7 are cut, in this embodiment by the cutting-of the last packing groove 3, the ferrous ring and the alloy will both move out so as to form an oval contour of the piston as shown in dotted lines Figure 3. It will, of course, be understood that the oval is exaggerated for illustrative purposes, because the actual distortion is small. The piston is now machined to size with the skirt of the usual round or oval contour. The piston will, however, remain round or oval (slightly) until in use in the engine; until then, the alloy will remain under tension, while the ring will remain under compression.

A piston of the character described and made as described can be made with a very low clearance and it will maintain a very close t when either hot or cold. That is for the following, reason: As the piston heats up, the head will of course, expand radially in all directions. Such radial expansion of the head will, however, only be transmitted to the'skirt in the region of the bosses. That will tend to relieve some of the strain along the wrist pin axis so` that the alloy and the .ferrous ring will .move out along that axis. That will cause the ring to tend to return to its original (viz. round or oval) form. In so doing, the strain in the alloy at the thrust faces is also relieved so as to move in with the ferrous ring at those faces. The piston, even one of an aluminum alloy, will therefore, remain conformed to the cylinder with a very low clearance, even below 0.00l-inch.

As an example of the piston-embodying this invention, the following are typical dimensions: a piston of an Al, 9.5 Cu, 0.5 Mg) aluminum copper magnesium piston alloy, having a diameter of 3.67-inches, has a ring of cold rolled steel 'of 0.125-in`ch diameter, exposed for onehalf of its extent, inside and `along-the thrust faces. This ring is close to the bottom ring groove and close to the outside of the skirt, viz., about equal to the thickness of the skirt.

In accordance withy this invention, the ring cast in the inside of the piston skirt at the upper end thereof, is anchored in the regions of the bosses with the anchorages extending somewhat into the-separated thrust face regions and exposed .for about onehalf of its cross section to the inside of the minor but substantial part of the thrust face regions. Byr separating the skirt from the head after solidification and cooling-of the metal on the ring, the cast metal and ring-stressed during such soliditcation and cooling, is relieved.

In the embodiment shown in Figure 7, a rib 15 is left inside of the ring 10, in order to provide for locating cont'act with apart of a mandrel. That does not however, detract from the flexibility of the skirt; for, although upon cooling of the piston alloy from solidiflcation to room temperature, the outside of the skirt will shrink and contract on the r-ing, the metal inside of the ring and at the rib 15, will shrink away from the ring. Accordingly, upon cutting'of the lowest packing groove 3, forming the circumferential slot or slots 7, the skirt will distort as indicated in Figure 3, and the operation of the piston will be substantially the same. That is because upon cooling of the alloy, the metal inside of the ring will contract away from the ring, so that it has no effect on the operation. That is because the contraction from the solidication temperature of the casting metal to room temperature, is so much greater than any expansion to the temperature of the piston in the engine, that the inside of the rib 15 will always clear the inside of the ring. Accordingly, the casting metal at the thrust faces will always be controlled by the ring engaging the inside of the thrust face. It will, therefore, be seen that the invention accomplishes its objects. A piston is provided which can be closely fitted to a cylinder of an internal combustion engine and which will maintain its dimensions from hot to cold so that there will be no seizing when hot and no slapping when cold.

We claim:

l. A trunk piston of an aluminum alloy, comprising, a head having a packing ring groove, a skirt provided with wrist pin bosses and directly attached to the outside of the head in both of the boss regions but axially separated from therhead along a thrust face region, and a continuous ferrous ring extending completely around the top of the skirt below its separation from the head, the ring being enveloped and cast-anchored by the alloy in both boss regions, which envelopment continues partially but extensively into the separated thrust face region, the inner part of the ring not enveloped by the alloy being exposed radially andcircumferentially medially along an extended part'of the separated thrust face region.

2. A trunk piston of an aluminum alloy, comprising, a head having a packing ring groove, a skirt provided with wrist pin bosses and directly attached to the outside of the head in both of the boss regions but axially separated from the head along a thrust face region, and a continuous ferrous ring extending completely around the top of the skirt below its separation from the head, the ring being enveloped and cast-anchored by the alloy in both boss regions, which envelopment continues partially but extensively into the separated thrust face region, the inside of the skirt being recessed medially along the separated thrust face region to expose the inner part of the ring radially and circumferentially medially along an extended part of the separated thrust face region, the recessing extending radially outwardly and axially to the bottom of the packing ring groove to eiect such separation of the skirt from the head.

3. A trunk piston of an aluminum alloy, comprising, a head having a packing ring groove, a skirt provided with wrist pin bosses and directly attached to the outside of the head in both of the boss regions but axially separated from the head along both thrust face regions, and a continuous ferrous ring extending completely around the top of the skirt below its separation from the head, the ring being enveloped and cast-anchored by the alloy in both boss regions, which envelopment continues partially but extensively into both separated thrust face regions, the inner parts of the ring not enveloped by the alloy being exposed radially and circumferentially medially along extended parts of the separated thrust face regions.

4. A trunk piston of an aluminum alloy, comprising, a head having a packing ring groove, a skirt provided with wrist pin bosses and directly attached to the outside of the head in both of the boss regions but axially separated from the head along both thrust face regions, and a continuous ferrous ring extending completely around the top of the skirt below its separation from the head, the ring being enveloped and cast-anchored by the alloy in both boss regions, which envelopment continues partially but extensively into both separated thrust face regions, the inside of the skirt beingV recessed medially along both separated thrust face regions to expose the inner parts of the ring radially and circumferentially medially along extended parts of the separated thrust face regions, the recessing extending radially outwardly and axially to the bottom of the packing ring groove to effect such separation of the skirt from the head in both thrust face regions.

5. The method of making a trunk piston of an aluminum alloy having a head for receiving a to-be-formed packing ring groove and having a skirt provided With wrist pin bosses directly attachedY to the outside of the head in both of the boss regions, comprising, positioning a continuous ferrous ring at the top of the skirt and over a pair of side cores and a three-part middle core one of whose outside parts extends radially and axially to the ring and to the bottom of the to-be-formed groove, casting the alloy over the core and over the ring in order to envelop and cast-anchor the ring in both boss regions, which envelopment continues partially but extensively into a thrust face region, leaving the inner part of the ring not enveloped by the alloy exposed radially and circumferentially medially along an extended part thereof, collapsing and withdrawing the core, and cutting the ring groove into the space vacated by the middle core in order to separate the skirt from the head in said thrust face region.

6. The method of making a trunk piston of an aluminum alloy having a head for receiving a to-be-formed packing ring groove and having a skirt provided with wrist pin bosses directly attached to the outside of the head in both of the boss regions, comprising, positioning a continuous ferrous ring at the top of the skirt and over a pair of side cores and a three-part core whose outside parts extend radially and axially to the ring and to the bottom of the to-be-formed groove, casting the alloy over the core and over the ring in order to envelop and cast-anchor the ring in both boss regions, which envelopment continues partially but extensively into the thrust face regions, leaving the inner parts of the ring not enveloped by the alloy exposed radially and circumferentially medially along extended parts thereof, collapsing and withdrawing the core, and cutting the ring groove into the space vacated by the core in order to separate the skirt from the head in both thrust face regions.

References Cited in the le of this patent UNITED STATES PATENTS 1,681,621 Nelson Aug. 21, 1928 1,784,291 Jardine Dec. 9, 1930 1,874,925 Diamond Aug. 30, 1932 1,891,914 Butlerv Dec. 27, 1932 2,119,137 Moore May 31, 1938 2,238,087 Bowser Apr. 15, 1941 2,426,732 Gates Sept. 2, 1947 2,551,488 Deming May 1, 1951 FOREIGN PATENTS 136,581 Austria Feb. 26, 1934 999,157 France Jan. 28, 1952 

